drm/imagination: Break an object reference loop

In the Linux kernel, the following vulnerability has been resolved:

drm/imagination: Break an object reference loop

When remaining resources are being cleaned up on driver close,

outstanding VM mappings may result in resources being leaked, due

to an object reference loop, as shown below, with each object (or

set of objects) referencing the object below it:

PVR GEM Object

GPU scheduler "finished" fence

GPU scheduler “scheduled” fence

PVR driver “done” fence

PVR Context

PVR VM Context

PVR VM Mappings

PVR GEM Object

The reference that the PVR VM Context has on the VM mappings is a

soft one, in the sense that the freeing of outstanding VM mappings

is done as part of VM context destruction; no reference counts are

involved, as is the case for all the other references in the loop.

To break the reference loop during cleanup, free the outstanding

VM mappings before destroying the PVR Context associated with the

VM context.